Lawyers for David Hicks say the Australian's terrorism conviction could be overturned after a landmark decision in a US court overnight.

A US appeals court has ruled that the charge Hicks was convicted of in 2007 - providing material support to terrorism - cannot be applied retrospectively.

The court has overturned the conviction of Osama bin Laden's former driver and bodyguard, Salim Hamdan, saying providing support for terrorism was not a war crime at the time of Hamdan's alleged conduct from 1996 to 2001, and therefore could not support a conviction.

Steven Glass, a partner at Gilbert and Tobin, represented Hicks in his case against the Commonwealth which wanted him to forfeit the profits of his autobiography under the Proceeds of Crimes Act.

He is looking at what the latest ruling could mean for Hicks, but says he has not yet had a chance to speak to his client.

"People should not be convicted for offences that didn't exist at the time that they engaged in the conduct, and that was going to be a key issue in the Proceeds of Crime case," he said.

Mr Glass is also looking at the implications of Hicks' imprisonment in Australia.

"He was detained in South Australia for about six months after he returned, and of course if he was never convicted, there could be an argument that that detention was unlawful," he said.

Dan Mori, the former US military lawyer who represented Hicks at his Guantanamo Bay hearings, has told ABC News Breakfast that the decision means that Hicks's conviction was unlawful.

"It [the charge of material support for terrorism] is null and void for conduct prior to 2006," he said.

"It is showing that what they set up was ineffective. They were trying to set up an ad hoc process after the fact instead of using the Federal Court system in the US that has been trying terrorism cases for years before and worked effectively.

"Unfortunately the military commission system was set up and it was rushed, and not very effectively, so now we see the problems with it.

'A matter for him'

Prime Minister Julia Gillard says it is up to Hicks to decide how to respond to the landmark US case.

"It's important to remember Mr Hicks was convicted under US law not under Australian law," she said.

"What Mr Hicks does in light of that case is a matter for him and whether or not that case is further appealed by US authorities is a matter for the US."

Hicks was captured in Afghanistan in 2001 and taken to Guantanamo Bay detention camp. In 2007 he was charged with and pleaded guilty to providing material support for terrorism.

He was sentenced to seven years' jail, all but nine months of which were suspended, and was transferred from Guantanamo to an Australian prison in 2007 to serve out his term, subject to a 12-month gag order.

Mr Mori said it was now up to Hicks's current lawyers to decide if they would formally seek to have his conviction overturned.

"I think it would be great for some official recognition that what he was put through was not fair and was not just," he said.

"I think that this court decision supports that position and hopefully it will act as some catalyst to getting some real closure and clearing his name officially."

Mr Mori said the decision was also an embarassment for the Australian government of the time, which went along with Hicks's trial at Guantanamo Bay.

"I don't think there is any real serious consideration that his conviction was valid but the real question why did the Australian Government allow this to happen to an Australian citizen?" he said.

Hicks's father Terry Hicks said he had not yet spoken to his son to give him the news.

"We have been saying all along that the laws were being retrospected [sic] so that they could carry on with this procedure of charging him with something," he told ABC News Breakfast.

"We have asked for inquiries into all this business and into other things but it's been sidelined, too hard basket, may get other people into trouble, whatever.

"I suppose if David's name is legally cleared that makes me feel a lot better. It will make David feel a lot better, and I think the people that have supported David over the years, they will be able to put their hands up and say, 'This is what we have all been working for'."

The US military prosecutor who faced off against Mr Mori at the Guantanamo tribunal, Morris Davis, has described the appeal court decision as a blow to the credibility of the military tribunal system.

But he said Hicks was facing a 'Catch-22' because, as part of his plea deal, he had waived the right to challenge his conviction.

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